1. Field of the Invention
The present invention relates to a second higher harmonic wave generating element (which is called an SHG element in the following description) used as a light source such as an optical pickup. The present invention also relates to a light source device for emitting a light beam and an optical pickup using this light source device.
2. Description of the Related Art
Recently, a compact and reliable blue light source having a long lifetime has been required. An SHG element attracts public attention as one approach to this requirement. This SHG element is provided by combining a semiconductor laser with a wavelength converting element.
For example, in a light source in an optical pickup for an optical disk, it is preferable to set the wavelength of an emitted light beam to be shorter so as to reduce the diameter of a converged light spot and increase a capacity of the optical disk. In the general SHG element, the light beam having a short wavelength of 473 nm is emitted as SHG light. Accordingly, the SHG element is considered to be suitable for a light source for the optical pickup. However, to increase the capacity of the optical disk, it is necessary to use an SHG element for emitting SHG light having a wavelength shorter than the wavelength of 473 nm.
The above general SHG element uses a Nd:YAG laser medium having an absorption band which is not so wide. Accordingly, it is necessary to strictly control temperature of the semiconductor laser by arranging a temperature controller to restrain a change in absorbing amount of the light beam and provide stable output characteristics of the semiconductor laser. Therefore, the SHG element is large-sized and cost thereof is increased.
In the general light source device, the semiconductor laser can directly emit LD light modulated at a high speed in the order of about several MH.sub.Z by modulating a driving electric current of the semiconductor laser in the order of about several MH.sub.Z at a high speed. However, a modulating frequency (or a modulating speed) of the Nd:YAG laser medium is limited and is of the order of about several KH.sub.Z by its fluorescence lifetime. Accordingly, no solid laser such as Nd:YAG is modulated at a high speed in the order of several MH.sub.Z even when the LD light modulated at a high speed in the order of several MH.sub.Z is incident to the Nd:YAG laser medium from the semiconductor laser. The SHG light finally emitted from a Nd:YAG laser resonator is not changed to a light beam modulated at a high speed in the order of about several MH.sub.Z, but is changed to a light beam modulated in the order of about several KH.sub.Z.
Therefore, in the general light source device, the diameter of a converged light spot can be reduced, but no light beam modulated at a high speed can be emitted from the light source device.